It is very easy to see why your arms are so much more developed than your
chest and that is because every exercise in your split is essentially working on
the muscles of your arms.
Compound exercises such as bench presses, rows, shoulder presses and lat-pull
downs have prime movers and synergists which are responsible for the joint
Even though you're working "chest" when you're doing bench presses, your
triceps are performing a significant portion of the work. You can see this
by looking at the motion of your elbows during a bench press as they extend
around 90 degrees. This 90 degrees extension is around the same as doing a
high cable triceps extension but the triceps are likely under significantly more
You explain on your chest day you're doing 9 sets of various bench presses.
This amounts to basically doing 9 sets of triceps extensions. The same
goes for the biceps when you're doing lat pull downs and rows. Back
exercises also work the long head of the triceps to some degree.
Building the Chest: Exercise Form is Essential
Like I mentioned before, every time you perform a chest press, your triceps
are lifting some of the weight. This can be exacerbated greatly by your
If you are using exercise form which is inconsistent with your goals your
current workout split could equate to basically doing 3 workouts of all arm
exercises with only 1 single set for the chest (flies).
In order to greater emphasize your chest during pressing exercises you must
keep your shoulders retracted during the motions. This involves forcefully
squeezing your shoulder blades together as shown in the photo.
If you haven't been doing this, it is likely the prime reason why your
synergists (arm muscles) are overdeveloped while your chest is underdeveloped.
You likely have synergistic dominance which is a postural distortion pattern in
which the muscle fibers of your prime movers are inactive and dormant because
they are not needed during the motions which are dominated by the synergists
(triceps, biceps, anterior deltoids).
Ways to Attempt to Improve your Proportions
You will never totally isolate your chest during any pressing exercises.
To isolate your chest you need to more chest flies emphasizing proper form to
isolate the chest.
Performing 9 sets of various presses is not the way to go if you want to
build your chest. A better option is to perform 3 sets of one angle of
bench press followed by various angles of chest flies on dumbbells and cables.
If what has been mentioned previously is not the case, you may simply just
have to lay off the arm exercises. Unless you're a competitive
bodybuilder, having an arm day is pointless. Give your upper body a day of
rest and work your legs which is far more beneficial to your whole body than
doing biceps curls and triceps extensions.
If you do indeed have synergistic dominance of your triceps, it may be due to
poor posture. If your shoulders are protracted (rounded forward) the first
step is to begin lengthening your pec muscles by static stretching. You
may need some time off to correct your posture before you efficiently build your
chest muscles. For more information check out Muscle Imbalances and Posture Problems.
Balance out your Routine
If you have been working out non-stop for years, take a rest. If you
haven't did a set of squats in a year, start doing a leg day. If you have
been laying off of cardio, start doing it.
Over working a muscle group can be an annoying proportion now when you're
young but it can turn into a debilitating problem as you get older. Your
muscles have the power to change the shape of your skeleton and over time
misalignments can cause joint problems. Working out is beneficial but it
can also cause problems if you don't do it smart.